It is known to use a solution of calcium chloride (a so-called calcium chloride brine) as a fluid for use in the course of oil-well drilling, both as a drilling fluid and as a workover fluid/completion fluid after an oil-well is in production. However, solutions containing calcium chloride as the only dissolved salt have a maximum practical density of about 1.35 g/cm.sup.-3 which may not be sufficiently high for drilling at great depths or into high pressure formations. To increase the density of calcium chloride solutions, it is known to include calcium bromide and/or zinc bromide but the zinc bromide component has acute environmental and economic disadvantages.
A further problem is that the low ambient temperatures encountered in oil/gas/hydrocarbon exploration in some areas of the world (e.g. the North Sea) can result in crystallisation of the calcium chloride from solutions containing more than 35% calcium chloride so that the fluid cannot be used.
U.S. Pat. No. 4,609,476 states that high density brine fluids for drilling operations may be solutions containing 10-60% by weight of salts such as CaCl.sub.2, NaBr, NaCl, KSCN, Ca(NO.sub.3).sub.2, ZnCl.sub.2, ZnBr.sub.2, CaBr.sub.2, KI, LiCl. CaI.sub.2, Ca(SCN).sub.2 and their mixtures having densities up to about 2.4 g/cc. Although one possible mixture which is covered by this statement would comprise calcium chloride and calcium nitrate, there is no specific disclosure of such a formulation in the patent and therefore no suggestion as to the relative proportions in which these two components would be used.